The goal of this project is to use quantitative light (LM) and electron microscopy (EM) along with other morphological and biochemical methods to study cellular mechanisms of myelin formation, breakdown and regeneration. This year, to compare nerve regeneration in young adult and aging mice, we transected sciatic nerves in 6- and 24-month-old mice. After 2 weeks, the nerves were removed, transverse sections were cut 5 mm distal to the transection site, and regeneration of myelinated and unmyelinated axons was studied quantitatively by LM and EM. The results showed that in sciatic nerves of aging mice, regeneration of myelinated axons was retarded. In these nerves there also were more Schwann cells that surrounded a single unmyelinated axon more than one micron in diameter, suggesting that ensheathment of axons by Schwann cells, a process that precedes the onset of myelin regeneration, also occurred more slowly during aging. Mechanisms responsible for the delayed myelinated fiber regeneration during aging are being explored in current experiments. Another important finding in these experiments was that regeneration of unmyelinated axons in nerves from young adult and aging mice did not differ significantly.